1. Field of the Invention
The present invention relates to a ladder filter including a plurality of series arm resonators and a plurality of parallel arm resonators and a duplexer.
2. Description of the Related Art
In recent years, ladder filters have been widely used as band-pass filters in cellular phones. Japanese Unexamined Patent Application Publication No. 5-183380 discloses a ladder filter including a series arm resonator and a parallel arm resonator which are surface acoustic wave resonators.
For example, duplexers for use in Band 13 are currently required to increase the amount of attenuation in a public safety band that is outside of a transmission band of Band 13 on a lower-frequency side by 2 MHz.
It is difficult for ladder filters in the related art to sufficiently attenuate a signal in a vicinity of a pass band on a lower-frequency side of the pass band.